Abaixo está uma instrução que descreve uma tarefa de geração de SQL, combinada com uma entrada que fornece mais contexto sobre os esquemas de tabela disponíveis. Escreva o código SQL que responda adequadamente à solicitação
 ###Instrução: Qual número de partida tem um campo de H e o oponente Chelsea sub-18?
 ###Tabelas: CREATE TABLE matches (matches_id AUTO_INCREMENT PRIMARY KEY, Match INT, Date VARCHAR, Competition or tour VARCHAR, Ground VARCHAR, Opponent VARCHAR, Score1 VARCHAR) CREATE TABLE cast_of_major_productions_1954–1999 (cast_of_major_productions_1954–1999_id AUTO_INCREMENT PRIMARY KEY, Character VARCHAR, 1954 Broadway VARCHAR, 1955 broadcast VARCHAR, 1960 broadcast VARCHAR, 1979 Broadway VARCHAR, 1990 Broadway VARCHAR, 1991 Broadway VARCHAR, 1998 Broadway VARCHAR, 1999 Broadway VARCHAR) CREATE TABLE kei_and_ki_indexes_by_country (kei_and_ki_indexes_by_country_id AUTO_INCREMENT PRIMARY KEY, Country VARCHAR, KEI VARCHAR, KI VARCHAR, Economic Incentive Regime VARCHAR, Innovation VARCHAR, Education VARCHAR, ICT VARCHAR, 2008 Rank INT) CREATE TABLE stations_under_construction (stations_under_construction_id AUTO_INCREMENT PRIMARY KEY, Station VARCHAR, Extension VARCHAR, City/Neighborhood VARCHAR, Projected opening INT, Transit Connections VARCHAR, Park and ride? VARCHAR) CREATE TABLE season_4:_2001-2002 (season_4:_2001-2002_id AUTO_INCREMENT PRIMARY KEY, Episode # INT, # in Season INT, Episode Name VARCHAR, Airdate VARCHAR, Location VARCHAR)  
SELECT AVG Match FROM matches WHERE Ground = h AND Opponent = chelsea under 18s